The following details of the new idea were provided by Benny Peiser and Michael Paine and appear here with only minimal editing for style and clarity:

The findings are calculated on the basis of the generally accepted "impact rate" (i.e. the rate of cosmic impacts calculated from terrestrial and lunar impact craters together with the currently observable flux of asteroids and comets in the solar system). A computer simulation of cosmic impacts over a 5 million-year period was chosen to give an indication of the environmental disruptions that have occurred during the evolution of our species.

These consequences can be categorized into:

A. Local -- devastation over a radius of tens of (miles) kilometers. No serious regional or global consequences

C. Severe regional -- devastation over 600 miles (1,000 kilometers) (the size of a large country). Severe regional climatic disruption. Mild, short-term global climatic disruption -- year without summer.

D. Moderate global -- devastation over thousands of miles (kilometers) -- continental. Severe global climate disruption lasting several years. Global food chain failures

E. Severe global -- global firestorms from ballistic entry of impact debris. Extreme worldwide climate disruption for decades to centuries. Extinctions.

For everything except the last category, the effects on early human populations depend on proximity to the impact -- a matter of luck. In addition to climate disruption (mainly darkness and cooling), the larger impacts could lead to global warming due to the greenhouse effect (water and carbon dioxide), loss of the ozone layer (particularly with ocean impacts that propel chlorine into the upper atmosphere), acid rain and toxins.

The simulation looked at the worst event in each of 5,000 millennia. It therefore gives an underestimate of the total number of impacts.

The program recognizes five outcomes of an asteroid or comet colliding with Earth:

• The object skims the atmosphere and flies harmlessly back into space. This happened in 2 percent of the millennia.
• The object explodes above land in an airburst similar to an atomic explosion. This happened in 17 percent of the millennia.
• The object impacts the land and forms a crater. This happened in 11 percent of the millennia.
• The object explodes in an airburst above an ocean. This was the most frequent outcome, accounting for 41 percent of millennia. Fortunately, until recently, most of these impacts would have been harmless to land dwelling creatures.
• The object impacts the ocean, forming tsunami and, possibly, ejecting vast quantities of water and salt into the atmosphere. This happened in 28 percent of the millennia. (Larger impacts may also reach the ocean floor and cause similar effects to a land impact)

Over the period of the simulation some 57 percent of millennia suffered an impact that would potentially have consequences for land-dwelling creatures. In most cases they would only be affected when they were close to the impact site. The situation is different now with significant human populations living in low-lying coastal areas.

Size impactor (The letters refer to the typical environmental consequences.)

1,650 to 2,950 feet (500 to 900 meters) (C): 108 events

0.6 to 0.9 miles (1 to 1.5 kilometers) (C/D): 24 events

1 mile (1.6 kilometers +) (D/E): 13 events

Craters (The letters refer to the typical environmental consequences.)

Over the 5,000 millennia a total of 552 craters were formed on land. Of these:

• 477 were less than 3 miles (5 kilometers) in diameter (A);
• 64 were between 3 and 6 miles (5 and 10 kilometers) in diameter (B);
• nine were between 6 and 12 miles (10 and 20 kilometers) in diameter (C);
• two were more than 12 miles (20 kilometers) (D).

There were also six ocean impacts that could be expected to produce moderate-to-severe global climate disruption (D/E), particularly destruction of the ozone layer. Three of these involved transient craters more than 31 miles (50 kilometers) in diameter and would probably have penetrated to the ocean floor.

Therefore, during this simulation severe climate disruption occurred, on average, every million years (i.e., two land impacts and three ocean impacts).

The findings by Peiser and Paine are underpinned by a significant number of large impact craters. So far, 32 impact craters have been discovered that are younger than 5 million years. One is 32 miles (52 kilometers) in diameter, three are between 6 and 12 miles (10 and 20 kilometers), one is between 3 and 6 miles (5 and 10 kilometers) and 25 are less than 3 miles (5 kilometers) in diameter. However, it should be noted that it takes very unusual conditions to preserve craters of this size for more than a few hundred thousand years.

Additional background material is provided by the authors here.

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